Phase locked loop oscillator circuits are in widespread use in a varied number of applications. Typical phase locked loop oscillator circuits include a voltage controlled oscillator, a phase detector, a source of reference signals and a loop filter. The voltage controlled oscillator produces an output signal having a frequency which is precisely controlled by referencing its phase to that of the reference frequency. The phase detector detects any phase error between the signal derived from the voltage controlled oscillator frequency and the signal derived from the reference frequency. The output of the phase detector is applied to the input of the loop filter the characteristics of which determine the characteristics of the phase locked loop. Finally the loop is closed by applying the output of the loop filter to a voltage control input terminal of the voltage controlled oscillator. By closing the feedback loop in this manner, the voltage controlled oscillator frequency tracks the reference signal frequency. When the phase of the signal derived from the voltage controlled oscillator frequencies properly tracks the phase of the signal derived from the reference frequency, the loop is in its locked operating condition.
The two signals applied to the phase detector often are not the same frequency at the start of operation or when a change in the tuning of a television receiver or the like is desired. When the signals are not of the same frequency, the phase difference between them constantly changes. This phase difference repeats itself once every 360 degrees of phase change, so that the output of the phase detector is an alternating current waveform which oscillates once for every 360 degrees of phase change. Various types of phase detectors are available, some of which produce a direct current component of the proper polarity for tuning the voltage controlled oscillator in the proper direction to ultimately establish lock-in of its operation, but even such phase detectors still have A.C. components in the outputs. Thus, the loop filter is necessary to smooth out these A.C. components and produce a direct current control voltage for the voltage controlled oscillator. Any A.C. components in the output of the filter which are not completely suppressed tend to frequency modulate the voltage controlled oscillator, which in most applications is an undesirable and detrimental condition.
As is apparent from the above description, the design of the loop filter is a key to the proper operation of the system. Necessarily, the loop filter characteristics are a compromise among different desirable characteristics of the operation of a phase locked loop. Loop filter designs which suppress all of the alternating current voltages from the phase detector outputs are often very slow in responding to frequency variations or changes between the two signals applied to the phase detector or phase comparator. For systems where sudden and fairly substantial changes in the frequency of the voltage controlled oscillator are desired, such as in television frequency synthesizer tuning systems, this slow response time is not desirable. A user of such a television receiver expects the receiver to nearly instantaneously tune to the newly selected channel and a slow pull-in or lock-in to a new channel prevents this from happening.
On the other hand, if the loop filter provides fast lock-up characteristics, the filter generally cannot suppress all of the alternating current voltages which are present in the output of the phase detector. Thus, even though relatively fast lock-up of the loop to the desired frequency takes place, continuous modulation or variation and change of the voltage controlled oscillator frequency is effected by the alternating current modulation permitted to pass from such a fast response time filter.
Attempts have been made in the past to overcome these seemingly incompatable demands placed upon the loop filter to modify phase locked loop systems to cause them to have relatively fast acquisition times and still have the stability provided by a loop filter otherwise exhibiting the characteristics of a slow response time. One such approach, is employed in the patent to LaFratta, U.S. Pat. No. 4,151,485, issued Apr. 24, 1979. In the LaFratta system, a digital clock signal is caused to track a pulse stream data signal by developing two phase-lock restorative voltages through a phase-locked loop circuit to control the loop voltage controlled oscillator which generates the clock signal. This is accomplished in the phase detector by generating a first voltage developed through an up/down counter and a digital-to-analog converter whenever the phase difference between the two signals exceeds a first threshold. A second voltage, which is designated as the coarse voltage is then generated by combining the fine voltage with a voltage which reduces or increases its value before application to the voltage controlled oscillator. As a result, the altered control voltage rapidly restores phase lock whenever the phase difference exceeds some second greater threshold. This system requires a relatively complex series of gates and delay circuits in the phase detector for accomplishing the desired result. The LaFratta system is not one which is specifically directed to the acquisition mode of operation of a frequency synthesizer type of phase locked loop system.
Another system of the prior art which is directed to an improvement in the acquisition time of a phase locked loop is disclosed in the Perkins, Jr., et al. U.S. Pat. No. 3,495,184, issued Feb. 10, 1970. This patent utilizes a pair of positive and negative slicing circuits connected to the output of the phase detector of the phase locked loop to produce additional pulsating signals which are summed into the output of the loop filter to control the voltage controlled oscillator. This system is limited in its effect since the gain of the system is necessarily reduced because of the pulsating nature of the additional control signals.
Another approach which has been proposed is one shown in the disclosure of the patent to Boelke, U.S. Pat. No. 3,611,175, issued Oct. 5, 1971. This patent primarily is directed to an effort to extend the captive or pull-in range of the phase locked loop system and does so with a complex implementation of an I.F. amplifier and a mixer into the loop in addition to the conventional loop circuit components. The mixer and the I.F. amplifier introduce additional signals into the loop system which require a relatively large number of components in addition to further bistable circuits and gating circuits to accomplish the purpose of extending the pull-in range of the circuit. Accordingly, it is desirable to provide a phase locked loop system suited for use in a frequency synthesizer tuning system which has improved acquisition time combined with loop filtering approaching ideal filtering for steady state conditions of operation.